Allow requests with extra_params and no parameters

Without this commit, the following request:

post '/orders' do
  example_request 'should take an optional parameter hash', order_type: 'big' do
     ...
  end
end

would **not receive** the POST body/param `order_type: 'big'` specified.

The reason is that the function `params` was returning nil if
example.metadata[:parameters] was undefined, and the only one to
define example.metadata[:parameters] was to have previously called
"parameter". In other words, requests were not allowed to have so-called
"extra parameters" unless they had at least one "parameter" specified.

This commit removes this restrictions, allowing requests to specify all
their parameters "inline".

lib/rspec_api_documentation/dsl/endpoint.rb

@@ -52,8 +52,7 @@ def query_string
     end
 
     def params
-      return unless example.metadata[:parameters]
-      parameters = example.metadata[:parameters].inject({}) do |hash, param|
+      parameters = example.metadata.fetch(:parameters, {}).inject({}) do |hash, param|
         set_param(hash, param)
       end
       parameters.merge!(extra_params)

spec/dsl_spec.rb

@@ -357,6 +357,18 @@
     end
   end
 
+  context "request with only extra params" do
+    post "/orders" do
+      context "extra options for do_request" do
+        before do
+          client.should_receive(:post).with("/orders", {"order_type" => "big"}, nil)
+        end
+
+        example_request "should take an optional parameter hash", :order_type => "big"
+      end
+    end
+  end
+
   context "last_response helpers" do
     put "/orders" do
       it "status" do